On 06/ 5/12 12:26 AM, Magnus Danielson wrote:
On 05/06/12 00:30, Dr. David Kirkby wrote:
This is not exactly a time related question, but I'm sure the subject
must be of interest to time-nuts using GPS.
If one transmits from an antenna such as a helical one, RHCP, can the
same antenna be used for reception, or does the helix need to be wound
the other way?
If you google this topic, there seems to be a lot of confusion about
whether the TX antenna and RX antenna need to both have RHCP or whether
one needs to be LHCP and the other RHCP.
Given GPS uses circular polarization, I'm hoping someone here will know.
It would appear there are different definitions of "circular
polarization", with one considering it from the point of view of the
source, and the other considering it from the point of view of the
receiver. The IEEE apparently uses the former, and others (especially
optics) use the opposite.
http://en.wikipedia.org/wiki/Circular_polarization
My aim was to make a gain measurement of two circular polarized
antennas. I have two identical antennas, but are unsure if the signals
should be received strongly, or whether theoretically no signal would be
received. (Of course in practice, one never achieves perfect
polarization, so there will always be a signal detected, even if
cross-polarized.
They would have to have opposite rotation.
The waveform rotation will follow the transmitter antenna into the
receiver antenna. The receiver antenna follows the same rotation that
the transmitter antenna has, it's just that the face each other, so when
you turn one of the 180 degrees such that they face the same direction
you would see that they are in fact rotated in opposite directions.
I'm sure the sat folks can confirm this.
Cheers,
Magnus
I can confirm that I'm 100% sure that the polarization of the two antennas needs
to be the same - i.e. both RHCP or both LHCP. I built two of them for RHCP, and
got appreciate gain.
Despite what other may say, there does seem to be a lot of confusion about this
issue, but I've satisfied myself by building them and testing the gain using a
VNA as the signal source and detector.
Dave
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